In conventional drilling operations it is often common to line the interior walls of a well with a liner (which is often a steel or other alloy pipe or tubular) in order to stabilize the wellbore and prevent collapse, particularly in loose or fractured formations. The space or annulus between the face of the formation and the liner may be packed with sand, gravel or other material. In other instances the liner is cemented in place through pumping cement or a cementatous-type material into the annulus. Upon setting, the cement serves to stabilize the position of the liner within the wellbore, helps to prevent a collapse, sloughing and cracking of the exposed formation face and can further help to prevent the escape of fluids from the formation. Where the wellbore passes through a formation that is saturated with water, cementing the liner in place can help to prevent the release of water from the formation into the well. In other instances cementing of the liner will help prevent the release of gas where the well passes through formations that harbour pressurized gas. Where there are multiple zones that are targeted for the production of fluid, in some instances it may be desirable to seal off particular zones through cementing the liner in place to permit the controlled and systematic release of fluid from particular zones in a particular sequence.
Typically where a liner has been cemented within a production zone once the cement has hardened and cured the well is reentered in order to perforate the liner and to allow for the extraction of production fluid. A variety of mechanisms and techniques have been developed in order to perforate a liner, most of which involve the lowering of a perforating gun into the tubular and using the gun to perforate the liner or well casing. Such guns typically utilize shaped charges that produce holes within the side of the liner having a relatively consistent size and shape. Others have proposed various other mechanical devices to punch or drill holes in liners once they are in place. Still others have devised liners themselves that are pre-fitted with small explosive charges that can be detonated after the liner has been connected in place in order to perforate a particular zone or section of the liner.
In other applications liners are pre-drilled at the surface with solid aluminium or similar plugs inserted into the holes, following which the liner is lowered and placed into position within the well. Once in the well the liner can be cemented in place, with the plugs retaining the cement slurry within the annulus formed between the formation face and the exterior surface of the liner. After the cement has set the plugs can then be removed, effectively creating a perforated liner. Removal of the plugs can be achieved mechanically, ultrasonically, or through packing off the section of the liner in question and introducing an acid to dissolve the material from which the plugs are made.
While the above described methods and devices have met with varying degrees of success, they all suffer from their own inherent limitations and difficulties. The formation of holes or perforations within a liner through the use of an explosive raises obvious concerns in the case of an oil or gas well where flammable or explosive materials may be present. Many of the currently employed methods require the lowering of relatively complex and expensive equipment into the well in order to mechanically, explosively or otherwise perforate the liner. Further, where the liner is cemented in place the mechanism used to perforate the liner or remove a plug that was previously inserted into a perforation is often incapable of removing or otherwise displacing the cement that has set up and hardened between the liner and the face of the formation. In such cases extracting fluid from the formation through the perforated liner can be difficult or inefficient.